An old 27, Adam is a neat freak with a tidy life – a solid job at a public radio station, a sound relationship with his live-in girlfriend, and a nagging backache that seems like nothing more than confirmation of his precocious maturity.

Or maybe not. A casual check-up brings the brutal news, delivered by an equally brutal doctor whose eyes never stray from the X-ray, the one that reveals a large and malignant spinal tumour. Straight off, then, Adam gets his survival odds and the movie gets its title: 50/50. Cue the comedy.

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Yes, the rest spins his potentially terminal cancer into a buoyant romcom, an unlikely but surprisingly agile partnership that dances smoothly between the pathos and the laughs.

Another surprise: You would expect the humour to be black, but it isn’t. Instead, the hues are much brighter and yet still believable, a credibility that can be traced to Will Reiser’s quasi-autobiographical script. When he too was in his 20s, Reiser received an identical diagnosis, which surely explains how the film manages to ground the laughter in the grit of hard experience. By turns silly and sombre, lively and deadly, the result is a rarity – not many reality checks are thoroughly entertaining.

Continuing with the autobiographical theme, Reiser has an off-screen friend who’s cast here in the same on-screen role: Seth Rogen plays Kyle, best buddy of the stricken Adam and a rich source of unsentimental yuks. Kyle is loyal but wonderfully awkward and shamelessly opportunistic, the kind of guy who’s constitutionally incapable of descending into emotional platitudes.

Given the sad news, girlfriend Rachael heads with suspicious speed towards a quick cliché: “I’m here for you.” Not Kyle, whose idea of comfort is to plaster a happy face on the 50/50 prognosis: “It’s not that bad. If you were a casino game, you’d have the best odds.”

In a sense, everything that follows is a continuing riff on the subject of easy labels and worn clichés, starting with the most obvious: In an instant, Adam (Joseph Gordon-Levitt) has morphed from healthy citizen to “cancer victim.” He’s entered the kingdom of the sick, where the disease becomes your calling card. “Hi, I’m Alan, stage 3 lymphoma” is his greeting from a fellow patient in the chemo ward.

Over in a counselling room, Katherine the “support care” specialist (Anna Kendrick) is well-meaning but, at 24, a raw rookie in the sensitivity biz. When she places her hand on his arm, his new status as a professional victim is sealed. Yesterday, such a touch from an attractive woman would have been a come-on; now it’s a treatment, and Gordon-Levitt’s face – his performance is superbly nuanced throughout – registers that seismic shift with a pained grimace.

Their sessions together are a recurring trope and a delicate pas de deux, shifting in tone (first comic, then serious) and in psychology too – one isn’t as old as he seems, the other not as young as she looks. And, for the broader laughs, there’s always Kyle, who’s wise enough to avoid those smug clichés yet shrewd enough to exploit them.

Check out a hilarious barroom sequence where he encourages Adam, in the wake of Rachael’s departure, to sashay up to fetching babes, flash his freshly shaven head, and play the sympathy card: cancer as a God-given ace in the game of getting laid. Cut to the bedroom where, of course, the victor is in no shape to enjoy the spoils – another lightning transition from wacky to poignant.

Speaking of which, Anjelica Huston submits an affecting cameo as a twice-burdened woman – her husband has Alzheimer’s, now her son has cancer. Still, the script doesn’t cut mother or son any slack. When Adam was healthy, he saw her just as an overbearing parent to be avoided, and his sickness has only reinforced this narrow view. So, in the midst of the comedy, the point is made: The victim of a cruel ailment can be wantonly cruel himself.

Not everything works that well. Rachael the wayward girlfriend is clumsily drawn (simultaneously too thin and too thick), and director Jonathan Levine tosses in an antic car crash that upsets the carefully crafted rhythm.

But that’s the clunky exception to an otherwise effortless ride. Indeed, this is the sort of smart commercial vehicle that toys with clichés even while inviting hardened cynics to embrace the oldest one in Hollywood’s book – yep, you will laugh, you will cry. Guaranteed, 50/50 will prove a huge crowd-pleaser – bet money on it at any odds.